Bullet Speed

This tutorial takes a shot at explaining how circuits can be used to measure things beyond the capacity of human senses.

The reliability and predictability of electronic circuits make possible highly accurate tools for measuring phenomena beyond the scope of human senses, such as the velocity of a bullet. This is demonstrated in the tutorial below.

In the circuit above (made up, in fact, of three distinct but overlapping circuits), a speeding bullet causes a capacitor to discharge during the very brief moment that it passes between two Tin-Foil Strips that are part of the circuit. By calculating (with the help of a galvanometer) the difference in the capacitor’s charge before and immediately after the bullet is fired, one can determine how much current the capacitor discharged in that moment. From this the bullet’s velocity can be derived. Here is a breakdown of how this works.

When you first open this Web page, current (depicted by yellow-colored electrons) begins flowing from the negative terminal of the Battery through Tin Foil Strip A, then comes to a split. Some of the current goes to the Capacitor and charges it until it reaches its capacity (a known value, measured in units called microfarads). In addition, some of the current from the battery veers off through Tin Foil Strip B toward the positive terminal of the battery, and continues doing so even after the capacitor finishes charging. If you missed any of this, hit the Reset button.

To get our "before" reading, click the Read Charge button. On the Galvanometer, observe the needle deflect all the way to the right.

Click on the Fire button. Out of the Gun Barrel shoots a bullet that cuts Tin Foil Strip A, breaking the circuit.

Electrons stop traveling between the two terminals of the battery when that circuit is broken. At the same time, a potential difference between the negative and positive plates of the capacitor is created. As a result, current flows from the capacitor, through Tin Foil Strip B, and through the Resistor (with a known resistance measured in units called ohms) on its way to the opposite plate.

The current stops when the bullet cuts the second tin-foil strip, breaking that circuit.

Now it’s time to get the "after" reading from the galvanometer. Click a second time on the Read Charge button. Observe that the needle does not deflect as far to the right, reflecting the diminished charge in the capacitor.

So, by subtracting the amount of capacity remaining in the capacitor from the initial capacitance, it can be determined how much charge was used during the time the bullet passed between the two strips. This information can be used in conjunction with other knowns – the distance between the strips, the resistance – to mathematically determine the speed of the bullet.

Thanks to our scientific advisors on this page, James Brooks and James Andy Powell, head electronics engineer in the MagLab's Instrumentation & Operations division.